Fuel cells generate electric power by electrochemically reacting a fuel gas containing hydrogen with an oxidant gas containing oxygen such as air through a polymer electrolyte membrane that selectively transports hydrogen ions. Fuel cells generally have a laminated structure in which a large number of unit cells are stacked. When operated, fuel cells produce heat as well as electric power. Thus, stacked cells need to be provided with a cooling plate every a few unit cells in order to keep cell temperature constant.
There is a need to humidify the fuel gas and the oxidant gas. Hence, polymer electrolyte membranes used in polymer electrolyte fuel cells need to be moistened sufficiently with water. If the cell temperature is too high, saturated vapor pressure increases and the water content in the polymer electrolyte membrane therefore decreases, thereby deteriorating cell performance. If the cell temperature is too low, due to generation of water on the oxidant gas side by cell reactions, condensation of water vapor hinders sufficient permeation of the oxidant gas, thereby impairing cell performance. Thus, the temperature of the fuel cell needs to be maintained within an optimal temperature range.
A stack comprising a large number of stacked unit cells is called a “fuel cell stack.” It comprises membrane-electrode assemblies and separator plates having gas flow channels formed in their surfaces. The membrane-electrode assemblies and separator plates are stacked alternately. The stack includes a current collector plate for collecting generated power and an insulating plate disposed on each end which are sandwiched by end plates.
Each unit cell is cooled by a coolant flowing inside the separator plate so that the cell is maintained at suitable temperatures. However, unit cells close to the end plates tend to have lower cell temperatures in comparison with the cells in the middle of the stack because of heat dissipation that takes place due to the temperature difference between the cells and the outside air.
When a fuel cell is not generating power, no heat is generated. The cell temperature therefore in the unit cells located close to the end plates is at or close to outside temperature which is substantially lower than the operating temperature of the cell. In such a state, if humidified fuel and oxidant gases are introduced in order to start power generation, condensation is likely to occur in the gas flow channels, in particular in those cells furthest from the inlet of the oxidant gas to the fuel cell. The occurrence of condensation hinders the respective gases from permeating into the cells, possibly causing a phenomenon of voltage instability during power generation.
Also, when the fuel cell is controlled such that the amount of power generated is lower than the rated output, less heat is evolved by cell reactions. Hence, similar condensation is likely to occur in the gas flow channels in the unit cells located close to the end plates. The temperature is also low in cells located at the central portion of the cell stack. Thus, there is a possibility, although not so large in comparison with the unit cells located close to the end plates, of output instability in the central cells.
Thus, it is necessary to control temperature such that the output of the unit cells close to the end plates does not decrease in any operation state regardless of the amount of power generated.
Such voltage instability of the unit cells located close to the end plates due to condensation could be eliminated by constantly circulating the coolant at high temperature. However, this method causes the temperature of the coolant to rise unnecessarily in the case where the fuel cell generates sufficiently large amounts of power, and therefore, large amounts of heat, so that the water content in the polymer electrolyte membrane decreases. This impairs the power generating capacity of the cell because when the temperature of the coolant is set high in an attempt to avoid the above-mentioned output instability caused by condensation and the amount of power generation is raised thereafter, the temperature of the unit cells in the center of the stack becomes too high due to heat evolution by cell reactions, so that the output of these cells decreases.